Microporous polyolefin membranes are useful as separators for primary batteries and secondary batteries such as lithium ion secondary batteries, lithium-polymer secondary batteries, nickel-hydrogen secondary batteries, nickel-cadmium secondary batteries, nickel-zinc secondary batteries, silver-zinc secondary batteries, etc. When the microporous polyolefin membrane is used as a battery separator, particularly as a lithium ion battery separator, the membrane's performance significantly affects the properties, productivity and safety of the battery. Accordingly, the microporous polyolefin membrane should have suitably well-balanced permeability, mechanical properties, dimensional stability, shutdown properties, meltdown properties, etc. The term “well-balanced” means that the optimization of one of these characteristics does not result in a significant degradation in another. As is known, it is desirable for the batteries to have a relatively low shutdown temperature and a relatively high meltdown temperature for improved battery safety, particularly for batteries exposed to high temperatures under operating conditions. High separator permeability is desirable because it generally results in high battery capacity. A separator with high mechanical strength is desirable for improved battery assembly and fabrication, and for improved durability.
The optimization of material compositions, stretching conditions, heat treatment conditions, etc. has been proposed to improve the properties of microporous polyolefin membranes. For example, JP6-240036A discloses a microporous polyolefin membrane having improved pore diameter and a relatively sharp pore diameter distribution. The membrane is made from a polyethylene resin containing 1% or more by mass of ultra-high-molecular-weight polyethylene having a weight-average molecular weight (“Mw”) of 7×105 or more, the polyethylene resin having a molecular weight distribution (weight-average molecular weight/number-average molecular weight) of 10 to 300, and the microporous polyolefin membrane having a porosity of 35 to 95%, an average penetrating pore size of 0.05 to 0.2 μm, a rupture strength (15-mm width) of 0.2 kg or more, and a pore size distribution (maximum pore size/average penetrating pore size) of 1.5 or less.
WO 2000/20492 discloses a microporous polyolefin membrane having improved permeability. The membrane contains fine fibrils made of polyethylene having Mw of 5×105 or more or a composition containing such polyethylene. The microporous polyolefin membrane has an average pore size of 0.05 to 5 μm, and the percentage of lamellas at angles θ of 80 to 100° relative to a membrane surface being 40% or more in machine and transverse cross sections.
In general, microporous polyolefin membranes consisting essentially of polyethylene (i.e., they contain polyethylene only with no significant presence of other species) have relatively low meltdown temperatures. Accordingly, proposals have been made to provide microporous polyolefin membranes made from mixed resins of polyethylene and polypropylene, and multi-layer, microporous polyolefin membranes having polyethylene layers and polypropylene layers in order to increase meltdown temperature.
WO 2005/113657 discloses a microporous polyolefin membrane having conventional shutdown properties, meltdown properties, dimensional stability and high-temperature strength. The membrane is made using a polyolefin composition comprising (a) a polyethylene resin containing 8 to 60% by mass of a component having a molecular weight of 10,000 or less, and a Mw/Mn ratio of 11 to 100, wherein Mn is the number-average molecular weight of the polyethylene resin, and a viscosity-average molecular weight (“Mv”) of 100,000 to 1,000,000, and (b) polypropylene. The membrane has a porosity of 20 to 95%, and a heat shrinkage ratio of 10% or less at 100° C. This microporous polyolefin membrane is produced by extruding a melt-blend of the above polyolefin and a membrane-forming solvent through a die, stretching a gel-like sheet obtained by cooling, removing the membrane-forming solvent, and annealing the sheet.
WO 2004/089627 discloses a microporous polyolefin membrane made of polyethylene and polypropylene comprising two or more layers, the polypropylene content being more than 50% and 95% or less by mass in at least one surface layer, and the polyethylene content being 50 to 95% by mass in the entire membrane. The membrane has relatively high permeability and high-temperature strength, as well as a relatively low shutdown temperature and relatively high short-circuiting temperature.
JP7-216118A discloses a battery separator formed from a porous film comprising polyethylene and polypropylene as indispensable components and having two microporous layers each with a different polyethylene content. The polyethylene content is 0 to 20% by weight in one microporous layer, 21 to 60% by weight in the other microporous layer, and 2 to 40% by weight in the overall film. The battery separator has relatively high shutdown-starting temperature and mechanical strength.
With respect to the properties of separators, not only permeability, mechanical strength, dimensional stability, shutdown properties and meltdown properties, but also properties related to battery productivity characteristics such as electrolytic solution absorption, and battery cyclability characteristics such as compression resistance have been disclosed as important. In particular, electrodes for lithium ion batteries can expand and shrink according to the intrusion and departure of lithium, and an increase in battery capacity can lead to larger expansion ratios. Because separators are compressed when the electrodes expand, the separators are needed which suffer little, if any, decrease in electrolytic solution retention by compression. However, when the separators are provided with larger pore size to achieve improved electrolytic solution absorption, the compression resistance of the separators decrease. Battery separators disclosed in any of JP6-240036A, WO 2000/20492, WO 05/113657, WO 04/089627 and JP7-216118A have insufficient electrolytic solution absorption and/or retention characteristics. Thus, microporous polyolefin membranes for battery separators are desired which have improved and well-balanced permeability, mechanical strength, heat shrinkage resistance, meltdown properties, electrolytic solution absorption, and compression resistance.